The present invention relates to a semiconductor photodetector device and a manufacturing technique thereof, and particularly to a technique effectively applied to an improvement in reliability of mesa-type photodetector devices employing compound semiconductors.
Japanese Patent Laid-Open No. 2001-177143 discloses a structure in which a mesa is formed on a crystal to be a substratum as a photodetector employing compound semiconductors and a periphery of the mesa is embedded by a crystal made of a suitable material and having a proper carrier density (hereinafter, this structure will be referred to as xe2x80x9cembedded mesa-typexe2x80x9d).
FIG. 21 is a cross-sectional view illustrating an embedded mesa-type semiconductor photodetector described in the above-mentioned document. According to a brief description of a manufacturing method of this semiconductor photodetector, at first, a buffer layer 402 made of an n-type InAlAs crystal; a multiplication layer 403 made of an n-type InAlAs crystal; a field control layer 404 formed of a laminated body of a p-type InAlAs crystal and a p-type InGaAs crystal; an absorption layer 405 made of a p-type InGaAs crystal; a cap layer 406 made of a p-type InAlAs crystal; and a contact layer 407 of a p-type InGaAs crystal are made to sequentially grow on a main surface of a substrate 401 through a MBE (Molecular Beam Epitaxy) method, and thereafter the contact layer 407, the cap layer 406, the absorption layer 405 and the field control layer 404 are etched to form a first mesa 408 on the substrate 401.
Next, a regrown layer 409 having substantially the same height as that of the first mesa 408, made of a compound semiconductor crystal and having a low impurity density is made to grow on the substrate 401, and thereafter the regrown layer 409 and a crystal layer that is a lower layer thereof are etched to form a second mesa 410 around the first mesa 408. Then, a protecting film 412 and electrodes 413 and 414 are formed on the substrate 401 and further an anti-reflection coating 415 is formed on the rear surface of the substrate 401, and thereby the embedded mesa-type semiconductor photodetector is completed.
The embedded mesa-type semiconductor photodetector having the above-mentioned structure has an advantage of being capable of reducing a dark current in comparison with a simple mesa-type semiconductor photodetector not provided with a regrown layer 409 because an electric field intensity of a pn junction (wherein an interface between the multiplication layer 403 and the field control layer 404 is a junction surface) is weakened by the regrown layer 409. Since mechanical strength of each chip is improved by providing, around the first mesa 408, the regrown layer 409 having substantially the same height as that of the first mesa 408, there is also an advantage of allowing bonding to be easily performed to a wiring substrate, or the like.
In steps of manufacturing the embedded mesa-type semiconductor photodetector described above, the first mesa is formed by laminating crystal layers made of a plurality of kinds of compound semiconductors on the substrate and by patterning these crystal layers. However, since some of these crystal layers also include compound semiconductor crystal containing Al such as InAlAs crystal (for example, a cap layer made of p-type InAlAs crystal), a stable natural oxidization film is formed on the surface of Al exposed to a sidewall of the first mesa by forming the first mesa.
By this, when the regrown layer is made to grow around the first mesa, defects or surface states are formed on the interface between the crystal containing Al and the regrown layer. Especially in the case where there are defects between the cap layer having a high impurity density and the regrown layer having a low impurity density, a current path is formed between both layers, so that the dark current in the photodetector becomes large. An increase in the dark current decreases characteristics of important receiver sensitivity, and so reliability of the photodetector is reduced and, in the case of being remarkably reducing, there are some cases of not functioning as a photodetector.
An object of the present invention is to provide a technique for reducing a dark current in a mesa-type photodetector employing compound semiconductors.
The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanied drawings.
Brief descriptions of representative ones among the inventions disclosed in the present application will be made as follows.
A semiconductor photodetector according to the present invention comprises: a pn junction formed by a compound semiconductor layer with first conductivity type formed on a semiconductor substrate and a compound semiconductor layer with second conductivity type formed on an upper portion of said compound semiconductor layer with first conductivity type; a first mesa that is formed in said compound semiconductor layer with second conductivity type and whose no bottom portion reaches said pn junction; a second mesa that is formed in a regrown layer around said first mesa, the regrown layer being made of compound semiconductor crystal surrounding said first mesa, and whose a bottom portion reaches at least said pn junction; and a groove that is provided in a vicinity of the boundary between said regrown layer and said first mesa and whose no bottom portion reaches said pn junction.
A manufacturing method of a semiconductor photodetector according to the present invention includes the following steps (a) to (e) which comprise:
(a) a step of making a compound semiconductor layer with first conductivity type grow on a semiconductor substrate, and of making a compound semiconductor layer with second conductivity type opposite to said first conductivity type, grow on an upper portion of said compound semiconductor layer with first conductivity type;
(b) a step of forming a first mesa having a predetermined shape on an upper portion of said compound semiconductor layer with second conductivity type, and of etching said compound semiconductor layer with second conductivity type located in a region not covered with said first mask, up to such a depth as not to reach an interface of said compound semiconductor layer with first conductivity type;
(c) a step of making a regrown layer made of compound semiconductor crystal grow around said first mesa;
(d) a step of forming a groove by etching a vicinity of the boundary portion between said regrown layer and said first mesa up to such a depth as to reach no interface of said compound semiconductor layer with first conductivity type; and
(e) a step of forming a second mesa in each upper portion of said first mesa and said regrown layer located therearound, and of etching said regrown layer located in a region not covered with said second mask and said compound semiconductor layer with second conductivity type located in a lower portion thereof up to such a depth as to reach at least an interface of said compound semiconductor crystal layer with first conductivity type, and of thereby forming, around said first mesa, a second mesa whose a part includes said regrown layer located in a region in which said groove is formed.
According to the above-mentioned means, by providing the groove in a vicinity of the boundary portion between a regrown layer and a first mesa, the compound semiconductor crystal containing aluminum and the regrown layer are separated in the compound semiconductor layer with second conductivity type owing to the groove, and thereby no current path is formed between both layers and so a dark current can be reduced.
FIG. 22 illustrates one example of dark current-voltage characteristics of a semiconductor photodetector provided with a regrown layer around the first mesa, wherein a symbol [A] indicates the case where the cap layer is made of InAlAs crystal, and a symbol [B] indicates the case where the cap layer is made of InGaAs crystal containing no Al for the comparative purpose. As can be seen from the drawing, when an applied voltage is, for example, 25V, [B] is of nA level and [A] is be of xcexcA level.